There are many operations downhole that require circulation or reverse circulation through a tool string. Almost as often the circulation valve needs to be operated between two positions so that, for example, it can be run in open to the desired location and then after the circulation is done, it can be closed again.
There are many types of circulation valves that are in use downhole. Some have an internal ported sleeve that is attached to a housing with a port through a j-slot mechanism. With this type of valve picking up and setting down weight gets the ports aligned or misaligned, as needed. These types of valves are less suitable for deviated wellbores where it is difficult to know if picking up and setting down has actually shifted the circulation valve or merely stretched the tubing string from a location near a wellbore deviation.
Other types of circulation valves involve the use of ever larger balls to move a circulation valve between its end positions. This design allows an initial smaller ball to land on a seat to pressure up to set another tool followed by a further pressure to move the valve to another position. In order to move the valve again to its initial position a bigger ball has to land on a bigger seat to, for example, shift a different sleeve. The initial ball is typically released as its seat shifts into a recess and opens up. Such seats can be made of collet segments that are held together in an initial position to allow pressure buildup on a seated ball and then the collet fingers in a groove can spread apart allowing the ball to go on through.
As an alternative, a different seat has been employed that simply enlarges as the ball is blown though it with pressure. It then stands ready to receive another ball that is larger for another operation.
A circulation valve with disappearing balls has been offered. The idea here is to use a seat that keeps its dimension so that it can accept a constant ball size. The idea is that the ball lands on the seat and permits whatever operation is needed and then just goes away from exposure to well conditions over time. The problem with this design is that the balls are rather soft and are prone to be eroded during delivery or even when on the ball seat itself and before the operation that depends on the ball sealing can be accomplished.
Other issues that have affected ball seats made of a series of collets is that the sealing happens on a series of abutting shoulders and in a downhole environment where debris can settle on the seating surface and reduce the chance for a good seal on the ball. Additionally, the collets have some gaps between them where some of the applied pressure creates a bypass flow that inhibits the desired shifting movement of a sleeve.
Recently a solution was proposed in U.S. application Ser. No. 11/583,678 filed Oct. 19, 2006 that is also assigned to Baker Hughes Inc. That solution featured a circulating sub that could be opened with a first ball for circulation and the circulation port closed with a second ball that was preferably of the same size as the first ball. In that design as each movement occurred, the ball causing the movement was ejected. Some customers did not want the first ball ejected before it was time to set a packer below. The release of the initial ball that opened the circulation port could lodge in the packer below and cause it to set prematurely. Apart from that concern, there was another concern of available momentum for the initial movement to allow full circulation port opening. On some occasions either because a bypass flow around the ball at the seat made of adjacent collet heads or because of a braking effect of O-rings being run past recently sheared pin remnants, the initial movement to fully open the circulation port would not complete before the initial ball was ejected.
The present invention improves the above design. It uses concentric pistons with the inner piston having a series of collet fingers that form a seat to accept a dart. The circulation sub is run in with the circulation port closed. The first dart lands on the collet heads and shifts both pistons in tandem to open the circulation port while retaining the dart. A second dart lands on the first dart and a higher seat on the inner piston. Pressure buildup on the second dart shifts the inner piston with respect to the shouldered outer piston to close the circulation ports and eject both darts to a catcher below. On the way, the darts can be used to set a downhole tool such as a packer before winding up in the dart catcher. The circulation subs are modular and can be used in stacks with progressively larger darts to operate subsequent modules located uphole. These and other aspects of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and associated drawings while recognizing that the claims define the full scope of the invention.